Not just a straight left. It's the left hand period. Manny can make it come around the side and hook ur head off. But yea its more of his feints which creates all types of openings for his incoming angles.

How can you get a straight punch so fast that it's hard for boxers to avoid? They ALL know it's coming, but yet they ALL get clipped and hit by it eventually. So it's not really a secret but the secrecy is how fast and powerful it is. For Mayweather he will also get clipped by it since orthodox fighters have a harder time blocking and countering it especially since the Philly shell style he uses isn't the best defense for it as it limits mobility going left.

What exercises and punching things do yoo do to get a great straight punch?

Floyd won't be using his philly shell for that fight. He isn't the greatest defensive fighter for nothing. He will use the best suited defense against what manny paquiao brings.